In the year 1998, a phenomenon in which a metal thin-film filter (a hole array filter) in which openings each with a size smaller than the wavelengths of visible light are arranged two-dimensionally and periodically transmits a wavelength component corresponding to the periodic pattern of the openings with high efficiency was discovered. Since then, various studies and application uses of the metal thin-film filter have been proposed (e.g. see Non-Patent Literatures 1 and 2).
In such a metal thin-film filter, the color and the wavelength component intended to be transmitted, the transmission characteristics, the light polarization dependence, etc. are controlled by the size, shape, and periodic pattern of the opening. The physical mechanism that determines the transmission characteristics of the metal thin-film filter is the point that an electromagnetic wave is transmitted by means of surface plasmon polaritons (SPPs) in which free electrons of the metal surface are coupled with the electromagnetic wave. Such a metal thin-film filter processed by periodic microfabrication is widely known as a plasmonic filter.
The plasmonic filter is a filter formed of a simple metal thin-film, and has advantages of high compatibility with a method of forming a fine structure on a two-dimensional plane, such as lithography, and high flexibility of the design of the optical characteristics. Hence, the hole array filter is used for, for example, color filters etc. of charge-coupled device (CCD) image sensors and complementary metal oxide semiconductor (CMOS) image sensors (e.g. Patent Literatures 1 to 4 and Non-Patent Literatures 3 and 4).